Integrating cavities are known. Integrating cavities are typically made with a spherical housing having diffusely reflective (95-100% reflectivity) inner walls. An intense beam of light (infrared, visible, or ultraviolet) is usually introduced into the integrating cavity through an input port. A typical input port is a hole or a slit formed in the housing. Such a hole or a slit becomes a defacto source of light that is being integrated by an integrating cavity. The light is diffusely reflected many times by the interior surfaces of the integrating cavity and is finally emitted through a small exit port of the integrating cavity. A typical exit port is a small hole or a narrow slit formed in the housing.
Integrating cavities are relatively bulky because they have a large, internal cross section (approximately 12 mm or more in diameter), making apparatii that incorporate such integrating cavities also large. The large cross section corresponds to a large (relative to the exit port area) total internal area of the integrating cavity and results in low efficiency (&lt;30%) of the integrating cavity. Efficiency is defined as the ratio of the amount of light exiting the integrating cavity to the amount of light entering the integrating cavity.
Further, for an integrating cavity with a given size total port area (input port area(s) plus exit port area), the larger the input port, the smaller is the efficiency of the integrating cavity. Thus, low efficiency also results when thermal radiation light sources (for example, tungsten filament lamps) or discharge type light sources (for example, xenon, metal halide discharge, or fluorescent lamps) are used to produce the desired beam of light. This is because these light sources are operated external to the integrating cavity, and thus require a large input port.
Individual LED elements and LED arrays can be used to illuminate an integrating cavity. U.S. Pat. No. 5,548,120 discloses an LED array with its LEDs facing into an input port (which is a slit in a housing of the integrating cavity). It also discloses that a plurality of holes could be made in the housing and the individual LEDs can then be placed to face into these holes. The integrating cavities disclosed in this patent have a large circular cross section (relative to array width) and are, therefore, quite bulky. Further, each integrating cavity configuration must be individually determined to efficiently mount a particular LED array internal to the integrating cavity. This must be done in such a way that the LED array assembly does not absorb light, because light absorption reduces the efficiency and brightness uniformity of the integrating cavity. The problems are compounded if LEDs of various wavelengths are required to achieve a specific color balance. This is because these LEDs of various wavelengths often come in different packages, thus requiring a different assembly for LED packages of different wavelength. This renders the optimum mounting of these devices in an integrating cavity very difficult and expensive.